Less flex is better. Use the minimum amount that still gives you the function and convenience you need. For big machines that are unlikely to move frequently, run rigid duct straight in and skip the flex hose.
For CNC routers, you'll want just enough so that the gantry doesn't overstretch the hose at any point on the XY axis.
For CNC routers, you'll want just enough so that the gantry doesn't overstretch the hose at any point on the XY axis.